1. Field of the Invention
The invention relates to a digital television repeater, and more particularly, to a phase compensation apparatus and method for a digital modulator.
2. Background of the Related Art
Generally, a digital television repeater is provided in a digital broadcasting system. The digital repeater performs the function of repeating a digital broadcasting signal to be transmitted to subscribers located in a radio wave shade area that the broadcasting signal does not reach or in a poor reception area. In other words, the digital television repeater receives a weak broadcasting signal transmitted from a digital broadcast transmitter, amplifies the broadcasting signal, and then transmits the amplified broadcasting signal through a desired channel frequency so that subscribers in the radio wave shade area or in the poor reception area can receive the broadcasting signal.
FIG. 1 is a block diagram schematically illustrating a partial construction of a conventional digital television broadcasting system. The conventional digital television broadcasting system includes a television broadcasting station 100, a digital broadcast transmitter 101 for receiving a digital broadcasting signal from the broadcasting station 100 and transmitting the digital broadcasting signal to subscribers, and digital television repeaters 102 and 103 for amplifying and repeating the weak digital broadcasting signal transmitted from the digital broadcast transmitter 101 to serve an radio wave shade area or areas.
If it is assumed that the digital broadcast transmitter 101 transmits the digital broadcasting signal through a frequency F1, the digital television repeaters 102 and 103 located adjacent to the digital broadcast transmitter 101 can repeat the digital broadcasting signal with a predetermined quality even though they use channels different from the channel used by the digital broadcast transmitter 101. Thus, they repeat the digital broadcasting signal using frequencies F2 and F3, respectively. Also, they may repeat a broadcasting signal from another digital television repeater (not illustrated) located adjacent to the digital television repeaters 102 and 103.
FIG. 2 is a block diagram schematically illustrating one of the digital television repeater shown in FIG. 1. The digital television repeater includes a receiving antenna 200 for receiving a weak digital broadcasting signal, a receiver 201 for converting the weak digital broadcasting signal received through the receiving antenna 200 to an intermediate frequency (IF) band signal, a transmitter 202 for modulating the IF digital broadcasting signal converted by the receiver 201 to a required frequency band signal, and a transmission antenna 203 for transmitting the digital broadcasting signal modulated by the transmitter 202 to the subscribers.
FIG. 3 is a block diagram schematically illustrating the transmitter shown in FIG. 2. The transmitter includes a modulator 300 for converting the digital broadcasting signal, which is composed of audio and video signals and transmitted from the receiver as an MPEG-II transport stream signal, to a required frequency band signal, a filter 301 for converting the digital broadcasting signal converted by the modulator 300 to a radio frequency (RF) signal, and a high-output amplifier 302 for amplifying the RF digital broadcasting signal outputted from the filter 301. A power supply 303 supplies a required power to the modulator 300, filter 301, and high-output amplifier 302.
FIG. 4 is a block diagram schematically illustrating the modulator shown in FIG. 3. The modulator includes a signal forming circuit 400 for changing an MPEG-II transport stream signal inputted from the receiver to a quadrature phase signal (hereinafter referred to as a Q signal), a first digital-to-analog (D/A) converter 401 for converting an in-phase signal (hereinafter referred to as an I signal) of the MPEG-II signal inputted from the receiver into an analog signal, a second digital-to-analog (D/A) converter 402 for converting the Q signal into an analog signal, and a quadrature modulation circuit 403 for modulating the I and Q signals to required band signals using two quadrature carriers.
The quadrature modulation circuit 403 comprises a first mixer 404 for multiplying the I signal, which is outputted from the first D/A converter 401 as an analog signal, by a sine wave carrier SinW.sub.c t, a second mixer 406 for multiplying the Q signal, which is outputted from the second D/A converter 402 as an analog signal, by a cosine wave carrier CosW.sub.c t, and a summer 407 for summing the I and Q signals modulated by and outputted from the first and second mixers 404 and 406.
The MPEG-II transport stream signal transmitted from the receiver is inputted to the first D/A converter 401 and the signal forming circuit 400, and the signal forming circuit 400 produces the Q signal by delaying the phase of the inputted MPEG-II transport stream signal by 90 degrees, and inputs the phase-delayed signal to the second D/A converter 402. The MPEG-II transport stream signal inputted to the first D/A converter 401 will be the I signal.
The digital I and Q signals inputted to the first and second D/A converters 401 and 402 are then converted to analog signals, respectively, and the converted analog signals are inputted to the quadrature modulation circuit 403. The first mixer 404 in the quadrature modulation circuit 403 modulates the input I signal by multiplying the I signal by the the sine wave carrier SinW.sub.c t having a frequency of a required band, and the second mixer 406 modulates the input Q signal by multiplying the Q signal by the cosine wave carrier CosW.sub.c t having a frequency of a required band. Thereafter, the modulated I and Q signals outputted from the first and the second mixers 404 and 406 are summed by the summer 407.
The conventional digital television repeater as described above, is commonly located on a high mountain or in a desolate place to serve the radio wave shade area or the poor reception area, and thus suffers from at least the following problem.
First, since the high mountain or the desolate place where the digital television repeater is located undergoes a great change in environment, such as temperature and humidity, respective elements constituting the transmitter deteriorate. Distortion may occur in the I and Q signals. The modulator then outputs the digital broadcasting signal including a phase error so that the subscribers cannot enjoy digital broadcasting service of a predetermined quality.
Second, since the digital television repeater transmits the digital broadcasting signal with a high output of more that 1 KW, the phase distortion in the modulator exerts a serious influence upon other adjacent channels.
Third, in the event that the modulator of the transmitter in the digital television repeater operates abnormally, the system operator should perform a manual compensation work using an instrument. However, considering that the digital television repeater is located on a high mountain or desolate place, a great loss of time and manpower results.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.